Sintered stick-shaped heater

ABSTRACT

A sintered pin heater which is made of a ceramic composite structure and which has an essentially enclosed insulating layer and an external conducting layer. The insulating layer being obtainable from 51-57 mass percent of Si 3 N 4 , 37-42 mass percent of MoSi 2 , 2.4-2.8 mass percent of Al 2 O 3 , and 3.2-3.6 mass percent of Y 2 O 3 . The conducting layer  13  obtainable from 38-42 mass percent of Si 3 N 4 , 53-58 mass percent of MoSi 2 , 1.8-2.0 mass percent of Al 2 O 3 , and 2.4-2.7 mass percent of Y 2 O 3 .

BACKGROUND INFORMATION

The present invention relates to a sintered pin heater which is made ofa ceramic composite structure and which has an essentially enclosedinsulating layer and an external conducting layer.

German Published Patent Application No. 35 12 483 describes a ceramicheating element made of Si₃N₄/MoSi₂ composites having a proportion of35-75 mole percent of Si₃N₄, the average particle diameter of the usedSi₃N₄ powder being twice as large as that of the used MoSi₂ powder. Theaverage particle diameter of the MoSi₂ powder is 2 μm or smaller.

However, the utilization of this powder combination leads tosatisfactory strengths only if an axial hot-pressing or a hot isostaticpressing process is used. However, this method has the disadvantage thata hard machining requiring considerable outlay must be carried outsubsequently.

German Published Patent Application No. 35 19 437 describes anelectrical, ceramic heating device, likewise using Si₃N₄/MoSi₂ powders,the electrically insulating part being made of powders whose averageparticle diameter is 1-50 μm. The conductive powder has the same or alarger average particle diameter than the insulating powder. Theconductive part of the heating device is designed in such a manner thatthe electrically conductive powder is not larger than half the averagesize of the electrically insulating powder. In this case, as well as inGerman Published Patent Application No. 35 12 483, this powdercombination leads to products having a sufficient strength only if anaxial hot-pressing or a hot isostatic pressing process is used with theabove-mentioned disadvantages.

German Patent No. 37 34 274 describes ceramic composites on the basis ofsilicon nitride, aluminum nitride, and β sialon in combination withsecondary phases from different silicides, carbides, borides, andnitrides of transition-metal elements. Depending on the secondary phasecontent, these materials possess selectively adjustable electricalproperties. The adjustable specific values for the electrical resistanceof these materials at room temperature lie between 1·10¹³ to 1·10⁻⁴ Ωcmand exhibit a positive dependence on the temperature (PTC effect). Thestrength level of these composites produced in this manner does not liebelow 200 MPa. The method used there for manufacturing highlyheat-resistant composites is to be considered a uniaxial hot-pressingwhich, in particular, has disadvantages with respect to the shaping ofbodies manufactured from these composites, as mentioned above. Furtherdisadvantages are that bodies made therewith can have anisotropicmaterial properties because of the pressing direction and that themethod is only usable as batch process, i.e., not as continuous process.Moreover, this method requires high temperatures and pressures.

Also described in German Patent No. 37 34 274 is the implementation of aceramic heater or a sheathed-element glow plug using Si₃N₄/MoSi₂composites having sintered-in metal wires as supply leads.

OBJECT AND ADVANTAGES OF THE INVENTION

The object of the present invention is to provide a pin heater having ahigh strength during whose manufacture a hard machining requiringconsiderable outlay can be omitted.

The object of the present invention is achieved by a sintered pin heaterwhich is made of a ceramic composite structure and has an essentiallyenclosed insulating layer and an external conducting layer, theinsulating layer being obtainable from 51-57 mass percent of Si₃N₄ withd₅₀ being preferably less than 0.7 μm, 37-42 mass percent of MoSi₂ withd₅₀ being preferably less than 2-5 μm, 2.4-2.8 mass percent of Al₂O₃with d₅₀ being 0.2-0.3 μm, and 3.2-3.6 mass percent of Y₂O₃ with d₅₀being preferably 0.5-1.0 μm, and the conducting layer being obtainablefrom 38-42 mass percent of Si₃N₄ with d₅₀ being preferably less than 0.7μm, 53-58 mass percent of MoSi₂ with d₅₀ being preferably less than 2-5μm, 1.8-2.0 mass percent of Al₂O₃ with d₅₀ being preferably 0.2-0.3 μm,and 2.4-2.7 mass percent of Y₂O₃ with d₅₀ being preferably 0.5-1.0 μm.

The electrically insulating material which forms the insulating layerhas a specific electrical resistance of 10⁵-10⁶. The electricallyconductive material which forms the conducting layer has a specificelectrical resistance of 1·10⁻³-5·10⁻³Ω.

It is generally known that, apart from the concrete chemical compositionof the composite materials, the electrical properties, are determined bythe specific particle-size ratios of the used powders. With regard toboth the used materials, their quantitative proportions, and inparticular, due to their average particle diameter, the specificselection according to the present invention enables the manufacture ofelectrically insulating and electrically conducting composites which,subsequent to sintering, have a 4-point bending strength of at least 500MPa at room temperature, and which remains nearly unchanged up to atemperature of 1000° C.

In particular, the use of the very fine, highly sinter-active Si₃N₄ rawmaterial having an average particle diameter of less than 0.7 μm, andthe use of the MoSi₂ raw material having an average particle diameter of2-5 μm, for both manufacturing the electrically insulating material andthe electrically conducting material, result in these particularlyadvantageous properties of the sintered pin heater.

Using the method described in German Published Patent Application No.197 22 321, it is possible for combinations of these materials to beprepared, shaped and gas-pressure sintered. The sintering process ismade up of at least two stages, the first sintering being carried outunder inert gas, and the last sintering being carried out under anitrogen partial pressure of 2-10·10⁵ Pa, the temperature of the firstsintering stage being lower than that of the last sintering stage. Inthis context, a pressure of atmospheric pressure and a maximumtemperature of 900° C. is preferred in the first sintering stage. In thelast sintering stage, a sintering temperature between 1700 and 1900° C.is preferred.

Moreover, the last sintering stage can be carried out at variabletemperature and/or variable nitrogen partial pressure in such a mannerthat, in the constitution diagram, the ceramic composite structurecontains the pure phases of the insulating component and of theconducting component.

Furthermore, the sintering can be carried out in a range of the nitrogenpartial pressure having an upper limit Y₁=log p (N₂) and a lower limitY₂=log p (N₂), where the upper limit Y₁ and the lower limit Y₂ areexpressed according to the following functions:

Y ₁=7.1566 ln(T)−52.719

and

Y ₂=9.8279 ln(T)−73.988,

T being the sintering temperature of ≦1900° C. and being input in ° C.In this context, the nitrogen partial pressure p (N₂) is indicated inbar.

For manufacturing the conducting composite component, powders are usedwhich have the same morphological properties as the powders used formanufacturing the non-conducting composite component.

The two composite components are preconditioned by grinding in a mixingmanner. Subsequently, injection-moldable polymer compounds made of therespective composite components are manufactured from a specialpolypropylene and cyclododecane, are kneaded under protective gas athigh temperature and are granulated by cooling while continuouslykneading. Using injection molding (CIM=ceramic injection molding),preferably using two-component injection molding, a ceramic body isformed from the polymer compound material which will constitue theconducting layer, and the other polymer compound material is injectedsubsequently in a second step.

In a first annealing step, the organic binder is removed (debindering),and a pre-sintering up to 900° C. under 10⁵ Pa nitrogen is carried out.The main sintering takes place under a defined N₂ partial pressure,which is varied with the temperature in such a manner that it lieswithin the range specified in claim 6. For example, argon can be used asan inert gas. By admixing the inert gas, the total sintering pressurecan be increased to values of up to 10⁷ Pa.

In the later sintered compact, the used, very fine powders produce avery homogenous, finely interlocked, Si₃N₄-matrix sintered structurehaving a fine distribution of MoSi₂ in Si₃N₄. Furthermore, the powdersused according to the present invention surprisingly result in the veryhigh 4-point bending strengths.

The pin heater can be used as a ceramic sheathed-element glow plug.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the particle-size distribution of an Si₃N₄ powder which canbe used according to the present invention. In this context, theparticle-size distribution is very narrow, having an average graindiameter of 0.58 μm and a BET surface area of 11 m²/g.

FIG. 2 shows the particle-size distribution of an MoSi₂ powder which canbe used according to the present invention. The average grain diameteris 4.55 μm and the BET surface area is 1.1 m²/g.

FIG. 3 shows an oblique side view of a ceramic heating element in theform of a sintered pin heater. The pin heater has an external conductinglayer 2 composed of the conducting component and an essentially enclosedinsulating layer 1 composed of the insulating component.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT Example 1

The manufacture of the insulating composite component was prepared bymixing 54 mass percent of Si₃N₄, 2.58 mass percent of Al₂O₃, 3.42 masspercent of Y₂O₃, and 40 mass percent of MoSi₂. The silicon nitride usedin the process had the particle-size distribution shown in FIG. 1, andthe used molybdenum silicide powder had the particle-size distributionshown in FIG. 2. The manufacture of the conducting composite componentwas prepared by mixing 40.5 mass percent of Si₃N₄, 55 mass percent ofMoSi₂, 1.94 mass percent of Al₂O₃, and 2.56 mass percent of Y₂O₃. Themorphological properties of the two composite components were identical.

Injection-moldable polymer compounds were manufactured from each of thetwo composite components; the polymer compound that has insulatingmaterial after sintering containing 12 mass percent of Polybond® 1001,and the polymer compound that has conducting material after sinteringcontaining 12.5 mass percent of Polybond® 1001. Polybond® 1001 is ahomopolypropylene grafted with 6% of acrylic acid (manufacturer:Uniroyal Chemical). The polymer compounds further contained 6 masspercent of cyclododecane. The components were kneaded under protectivegas at 180° C. Subsequently, granulation was carried out by coolingwhile the kneading machine was running. The pin heater shown in FIG. 3was formed by two-component injection molding. Subsequently, theinjection-molded object was compacted by a gas-pressure sintering methodin the above-described manner.

The conductivity of external conducting layer 2 made of Si₃N₄/MoSi₂composite was 2.2·10⁻³ Ωcm; that of essentially enclosed insulatinglayer 1, which is likewise composed of an Si₃N₄/MoSi₂ composite, was1·10⁶ Ωcm.

Example 2

The manufacture of the insulating composite component was prepared bymixing 57 mass percent of Si₃N₄, 2.6 mass percent of Al₂O₃, 3.5 masspercent of Y₂O₃, and 37 mass percent of MoSi₂. The silicon nitride usedin the process and the molybdenum silicide powder were identical tothose from example 1. The manufacture of the conducting compositecomponent was prepared by mixing 38 mass percent of Si₃N₄, 57.8 masspercent of MoSi₂, 1.8 mass percent of Al₂O₃, and 2.4 mass percent ofY₂O₃. The respective composite components, which were preconditioned bygrinding in a mixing manner, were granulated with suitable auxiliarypressing agents, such as polyvinyl alcohol, in order to manufacturepressable powders. The pin heaters shown in FIG. 3 were formed bycomposite pressing, including subsequent green processing. Thecompaction was carried out analogously to example 1.

The 4-point bending strength at room temperature was 640 MPa±20 MPa forthe insulating component, and 600 MPa±25 MPa for the conductingcomponent.

What is claimed is:
 1. A sintered pin heater, comprising: a ceramiccomposite structure; an essentially enclosed insulating layer; and anexternal conducting layer, wherein a composition of the insulating layercorresponds to: 51-57 mass percent of Si₃N₄, 37-42 mass percent ofMoSi₂, 2.4-2.8 mass percent of Al₂O₃, and 3.2-3.6 mass percent of Y₂O₃,and wherein a composition of the conducting layer corresponds to: 38-42mass percent of Si₃N₄, 53-58 mass percent of MoSi₂, 1.8-2.0 mass percentof Al₂O₃, and 2.4-2.7 mass percent of Y₂O₃.
 2. The pin heater accordingto claim 1, wherein the composition of the insulating layer correspondsto: 51-57 mass percent of Si₃N₄ with d₅₀ being less than 0.7 μm, 37-42mass percent of MoSi₂ with d₅₀ being 2-5 μm, 2.4-2.8 mass percent ofAl₂O₃ with d₅₀ being 0.2-0.3 μm, and 3.2-3.6 mass percent of Y₂O₃ withd₅₀ being 0.5-1.0 μm, and wherein the composition of the conductinglayer corresponds to: 38-42 mass percent of Si₃N₄ with d₅₀ being lessthan 0.7 μm, 53-58 mass percent of MoSi₂ with d₅₀ being 2-5 μm, 1.8-2.0mass percent of Al₂O₃ with d₅₀ being 0.2-0.3 μm, and 2.4-2.7 masspercent of Y₂O₃ with d₅₀ being 0.5-1.0 μm.
 3. The pin heater accordingto claim 1, wherein the pin heater is produced according to a processinvolving: performing a first sintering under an inert gas, andperforming at least a second sintering under a nitrogen partial pressureof 2-10·10⁵ Pa, wherein: a temperature of the first sintering is lowerthan a temperature of the at least second sintering.
 4. The pin heateraccording to claim 3, wherein; a maximum of the temperature of the firstsintering is 900° C., and the first sintering is performed atatmospheric pressure.
 5. The pin heater according to claim 3, wherein: atemperature of the at least second sintering is between 1700 and 1900°C.
 6. The pin heater according to claim 3, wherein: the at least secondsintering is performed in accordance with at least one of variabletemperatures and variable nitrogen partial pressures such that theceramic composite structure contains pure phases of the insulating layerand of the conducting layer.
 7. The pin heater according to claim 6,wherein: at least one of the first sintering and the at least secondsintering is performed in a range of one of the variable nitrogenpartial pressures having an upper limit Y₁ and a lower limit Y₂, theupper limit Y₁ and the lower limit Y₂ are expressed according to thefollowing functions: Y ₁=log p(N ₂)=7.1566 ln(T)−52.719, and Y ₂=log p(N₂)=9.8279 ln(T)−73.988, T corresponds to a sintering temperature of≦1900° C., and p (N₂) corresponds to an input denoted in units of bar.